This invention generally relates to electric discharge lamps and has particular reference to an improved fluorescent lamp which has light output of a selected color and can be readily manufactured and started.
Fluorescent lamps which are designed to emit light of a predetermined color are well known in the art and achieve the desired color-controlled light output by employing suitable combinations of phosphors and light-filtering materials. For example, in the case of a red-emitting fluorescent lamp designed for use as a light source for a photographic dark room or as a decorative lamp, a phosphor coating that emits mainly in the red region of the spectrum in response to the ultraviolet radiations produced by the arc discharge is combined with an underlying layer of red pigment that filters out all of the visible radiations below a certain wavelength (600 nm, for example). While such filtering produces a corresponding reduction in the total light output of the lamp, the radiations which are transmitted provide the pure red light which is desired. A prior art fluorescent lamp which utilizes this color-controlling concept and combines a suitable phosphor and a filter material to provide a lamp which emits mainly in the yellow and red regions of the spectrum and thus serves as an insect repellent lamp is disclosed in U.S. Pat. No. 2,838,707 issued June 10, 1958 to Schwing et al. The light-filter in the prior art lamp comprises a layer of cadmium sulphide which underlies the phosphor coating and is admixed with finely divided silicon dioxide that serves as a starting aid and permits the lamp to be operated as a "quick starting" lamp.
A fluorescent lamp that produces gold-colored light by combining a pink-emitting phosphor (such as zinc beryllium silicate) and a filter layer composed of a pigment such as cadmium sulfide that contains up to about 50% by weight of barium sulfate is disclosed in U.S. Pat. No. 2,299,720 issued Oct. 20, 1942 to Holman.
The use of a transparent film of conductive material such as tin oxide on the inner surface of a fluorescent lamp envelope to facilitate lamp starting is also well known in the art. Low-pressure discharge lamps having such starting aids are disclosed in U.S. Pat. Nos. 3,624,444 (Berthold et al.) and 4,129,802 (Vrenken).
In order to avoid the lamp-starting problems which inherently result from the use of thin layers of pigment material in fluorescent lamps to filter out light rays of undesired colors, the prior art practice was to include various additives in the phosphor paint to facilitate starting of the finished lamp. In the case of red-emitting fluorescent lamps that utilize a thin layer of cadmium-selenium sulfide pigment as the filter component together with a phosphor which has an emission mainly in the red portion of the spectrum, a prior art practice was to add a mixture of barium sulfate and silica to the phosphor paint as a starting aid in amounts that frequently exceeded 40% by weight or more of the phosphor coating. Since such additives are inert (non-fluorescent) they significantly reduce the "red light" output of the lamp and, more importantly, do not completely solve the lamp-starting problem.